Predicting behavior of concrete through preliminary analysis of concrete properties is important for construction operation decisions. Conventional approaches for evaluating properties of concrete in the field are often inaccurate, destructive, time consuming, and costly. Two important ways of evaluating concrete are measuring the water-cement ratio of the concrete mixture prior to placement and measuring the compressive strength of the concrete at certain introductory time intervals to predict long term strength and durability.
Water-cement ratio is an important property for concrete strength and durability so getting reasonably accurate measurements of it for freshly mixed concrete is of great practical importance. The current field practice to estimate water-cement ratio is from batch mixture quantities, i.e., water-cement ratio is calculated from the amount of water and the amount of cement added to the mixture. The estimated results are not generally found accurate, as the quantities do not accurately account for factors such as moisture contained in aggregate or water added at the job site.
Given the practical importance of water-cement ratio, many techniques have been proposed to measure this property. These generally involve measurement of water content and measurement of cement content. Field experiences indicate that accurate measurement of the water-cement ratio of fresh concrete is very difficult to achieve and currently there are no fast and reliable technologies available. For example, the nuclear gauge, which is widely used for quality control of soil compaction, is found ill suited for field water-cement determination use due to its unsuitability for igneous aggregates, poor field performance with limestone aggregates, and extensive training and certification procedure for operators. Several ultrasonic research tests have also been proposed. However, although the research unveiled some potential application of ultrasonic technology in fresh concrete characterization, none of the approaches produced satisfactory solutions for the instantaneous determination of the water-cement ratio. Accordingly, there still remains a need for a technique that more accurately measures the water-cement ratio in a shorter period of time to improve quality control and quality assurance of concrete.
Compressive strength of concrete is an important factor that controls the service life of concrete structures. Measurement of concrete strength development is important for quality assurance and quality control. The direct way to evaluate concrete strength is from laboratory compression tests performed on batch samples obtained in the field. The process is time consuming and the strengths obtained are generally not representative of the behavior of concrete in the actual structure due to difference in curing conditions. It is not uncommon to obtain significant scattering of measured concrete strength from compression tests on cylinders. Besides, the tests can only be performed to obtain concrete strength at fixed time since the tests are destructive.
Alternative approaches exist which are based on correlating concrete strength to other properties that can be measured more easily and nondestructively. An important characteristic of concrete strength is that it increases with curing time, which is attributed to the continuing hydration process. Thus, characterization of the hydration process provides an alternative approach for determining concrete strength. Various indirect methods can be used for this purpose, including calorimetry, thermal techniques, and nuclear magnetic resonance spectroscopy, which are generally expensive and require sophisticated methods of data analysis. Alternative approaches, such as ultrasound, are based on measurement of ultrasonic velocity, which is more related to concrete modulus and was applied to study the initial setting of concrete. There are also other empirical approaches based on the relationship between concrete strength and porosity. All these approaches, while working under certain situations, have shortcomings with respect to field applications. Accordingly, there still remains a need for a technique that accurately predicts compressive strength of concrete within a short period of time after placing that is non-destructive and economical.